Age-dating helps in search for orebodies

The mention of age-dating may raise a few chuckles in some circles. But it really doesn’t have anything to do with pensioners going out with one another. Age-dating is the serious business of determining how old a given series of rocks is, then estimating the odds of that particular region hosting one or more orebodies.

A rock’s age, of course, is not the only data needed. As much information as can be garnered from field work, from the re-examination of previous work, from satellite imaging if it is available, is required. It is all grist if there is to be a mill; age-dating is only a single piece of information. There are two fundamentally different types of age so far as the economic geologist is concerned.

The first is vital to the petroleum geologist. It is based on certain fossil species being specific to a given rock formation. The age arrived at is a relative one.

When geology first came to be studied systematically, it was found that different ages of sedimentary rock were the graveyards for different families of organisms. A sequence of rocks several hundred feet below surface would be represented by a family of fossils quite different from the family within those rocks closer to surface.

The change in species reflected the change in climate as the rocks were laid down. Thus, the several hundred feet of rock may have taken hundreds of thousand of years to accumulate. The climate may have changed from equatorial to polar in that time with a subsequent change in vegetable and animal life. In geology, the usefulness of comparing fossils was soon recognized. A given fossil has the same geological age wherever in the globe it is found. It is as unique as a fingerprint.

But what can be done if the rocks are not sedimentary? Some of the world’s prolific ore-bearing regions are composed of once molten volcanics and igneous intrusions. This environment did not support life and if there was no life there will be no fossils.

Other orebodies are within sedimentary strata so severely altered by metamorphism their fossil content no longer exists.

Until the late 1940s, little could be done. Then, nuclear physics arrived. Most of the early research work was profoundly academic. But scientists were feeling their way through a minefield. There was a great deal of learning to do. They were pioneering a new discipline.

Today, a company can submit a sample to the Jack Satterly Laboratory for age-dating (located at the Royal Ontario Museum in Toronto; it is renowned as the world’s foremost age-dating laboratory) and have the result in two weeks. A rock may have an age of two billion years (plus or minus 1-2 million years). This is the actual time that has elapsed since the rock crystallized from the molten state and came into being.

Radioactivity is the key. Radioactive minerals break down into two or more components at a consistent and predictable rate. The rate is known. The amount of decay (breakdown) products in the sample is measured and so is the amount of unaltered mineral. By comparing the two, the time when the mineral first crystallized can be deduced.

The process is akin to estimating the time that has passed since flipping over the kitchen hourglass. If the upper bulb is one-quarter full (representing unaltered radioactive mineral) and the lower three-quarters full (representing decay products), then 45 minutes have elapsed. The most commonly used radioactive clocks are minerals containing uranium, rubidium or potassium. The last two are infinitely less radioactive than uranium but they are the usual components of most rock-forming minerals. Their radioactivity is faint but adequate for the age-dating process. (Rubidium decays to strontium, potassium to argon and uranium to lead and helium.)

Uranium-lead dating is most frequently used on rocks older than two million years; potassium-argon dating is used for those between 10,000 and three million years (although it is most reliable on rocks younger than a few million years). Rubidium-strontium overlaps both fields.

Carbon-14 dating is known to the general public from the revelations of the archeologist and historian. It is effective down to about 50,000 years from the present time. It is a vital tool for the geologist unravelling the globe’s latest glaciations.